Water adsorption on activated carbons:: Study of water adsorption in micro- and mesopores

The mechanism of water adsorption is analyzed in a range of activated carbons differing widely in the relative proportions of narrow and wide micropores, mesopores, and macropores. They also display different degrees of surface functionality. By assuming that the adsorbed phase in micropores is "ice-like", it is shown that the whole micropore volume is filled with water at a P/P-0 of about 0.82, at which point capillary condensation may begin. However, surface complexes control the wetting of the mesopore walls by adsorbed water and can influence the condensation process. Condensation in macropores may also occur at P/P-0 values above 0.95. The extent of meso-/macropore filling depends on the surface complexes. The volume filling process is progressive and in micropores is consistent with the theory of volume filling. The adsorption process was analyzed by interpreting the nitrogen isotherms systematically measured on samples of different pore structure with progressively increasing amounts of preadsorbed water up to saturation.